% \documentclass[UTF8,oneside]{ctexbook}
% \usepackage{amsmath}
% \begin{document}

\begin{flalign} % 左对齐
&\nonumber
\hspace{-0.7em}
    \begin{array}{cccc} % 说明文字
        % 第一行
        Method
            & \quad URL
            & \quad\,Parameter\left(s\right)
            & Protocol Version
        \\ % 第二行
        \downarrow 
            & \downarrow
            & \quad\,\downarrow 
            & \downarrow
    \end{array} \\
&\nonumber
    \hspace{0.75em} % Post
    \texttt{POST /comments/create?photo\_id=246 HTTP/1.1} \\
&\nonumber
    \left. 
    \begin{array}{l} % 包头
        \texttt{Host:\ www.example.com}  \\
        \texttt{User-Agent:\ Mozilla/5.0} \\
        \texttt{Accept:\ text/html,\ */*}   \\
        \texttt{Accept-Language:\ en-us}  \\
        \texttt{Accept-Charset:\ lSO-8859-l,utf-8} \\
        \texttt{Content-Length:\ 40}  \\
    \end{array}
    \hspace{4em}
    \right\} Headers \\
&\nonumber
    \hspace{0.75em} % body
    \texttt{comment=what+a+cute+baby\%21\&priority=low} 
    \longleftarrow Body
\end{flalign}

% \end{document}